The present invention relates generally to wavelength tunable distributed feedback (DFB) laser arrays, such as those to be used in fiber optic communication systems. The present invention relates more particularly to a photonic integrated circuit (PIC) comprising a DFB laser array and an active waveguide coupler, which facilitates the high power at the output facet.
Transmission through fiber optic communication systems is well known. In such fiber optic communication systems, light is modulated with desired information, such as voice, video or data, and is transmitted via optical fibers.
One inherent advantage of such optical fiber communication systems is the ability to transmit large amounts of information over each optical fiber. As the number of users of such systems increases, and as the quantity of data to be transmitted by each user increases, it becomes necessary for the fiber optic communication system to accommodate the transmission of ever-greater quantities of information.
In some instances, it may be cost effective to deploy additional optical fibers, so as to increase the capacity of the fiber optic communication system. However, in many instances, it is extremely expensive to deploy new optical fiber. In these instances, it is preferable to enhance the efficiency of wavelength utilization in order to increase the quantity of information transmitted via a single optical fiber.
In wavelength division multiplexed (WDM) systems, light having a plurality of different wavelengths is used. Each wavelength of single DFB is independently modulated directly or indirectly via modulator, so as to define a channel, which may be used to transmit information independently with respect to all other channels on the same optical fiber.
Thus, in order to provide wavelength redundancy, a tunable or wavelength selectable DFB laser is useful. One such wavelength selectable laser is an array of DFB lasers, the output of which is combined at one output.
Different methods for combining the outputs of active pumped DFB lasers are known. These methods can be broadly classified into discrete combining methods and broad combining methods.
Discrete combining methods select which wavelengths are combined so as to multiplex the outputs of a plurality of lasers utilizing thin-film filters or an arrayed waveguide device. Such wavelength-based couplers are only suitable for combining the outputs of a plurality of lasers when the wavelength of each laser is substantially fixed.
When the wavelengths to be combined onto a single optical fiber are variable, e.g., are not substantially fixed, predefined wavelengths, then a power based wavelength coupler must be utilized. In a power based wavelength coupler where the outputs of a plurality of lasers are added to one another in more of a brute force manner.
One example of a power based wavelength coupler is a Y-coupler, wherein passive waveguides are used to combine the outputs of DFB lasers, each having a different output wavelength, into a single composite beam. Such Y-couplers merely comprise waveguides which come together in a Y configuration, such that the two light beams transmitted therethrough are forced together. Any desired number (subject to transmission and coupling losses) of such Y-couplers may be used so as to facilitate the combining of any desired number of laser outputs.
Such contemporary Y-couplers contain only passive material, and thus, cause several inherent problems, as discussed in detail below.
An improvement to the basic Y-coupler is the contemporary multimode interference (MMI) coupler. The multimode interference (MMI) coupler, like the Y-coupler, is a passive power coupler. However, the multimode interference (MMI) coupler has the advantage of smaller size and better uniformity of output power across the entire range of wavelengths accommodated thereby.
Typically, an actively pumped semiconductor optical amplifier (SOA) waveguide is provided after the power based wavelength coupler (whether it is Y or MMI coupler). The actively pumped SOA waveguide tends to compensate for power losses inherently introduced by these power based wavelength coupler.
However, disadvantages commonly associated with the use of such power based wavelength couplers is the alignment of the passive unpumped coupler with the active pumped DFB lasers and active pumped SOA waveguide. Thus, substantial processing and yield problems, particularly with respect to integration of the active and passive components, occur according to contemporary methodology.
Additionally, the passive region of such contemporary devices is unpumped and thus requires the use of a very low absorption loss material. Therefore, the vertical structure of the passive region cannot be the same as the vertical structure of the DFB laser and SOA waveguide. This introduces the additional disadvantage that fabrication of such a contemporary active-passive-active device requires two distinctly different growth processes, as well as an undesirably large number of lithography and etching steps. Moreover, the use of such growth processes and such lithography and etching process steps undesirably complicates and increases the cost of fabricating such contemporary devices.
A further disadvantage of such contemporary devices involves the interface between the active and passive regions thereof. Optimization of this interface, so as to achieve suitably low coupling loss and good alignment, is very difficult. Even the best interfaces achieved according to contemporary methodology lead to a small refractive index difference and therefore inherently cause unwanted reflections that degrade DFB lasers"" performance.
In view of the foregoing, it would be beneficial to provide a PIC which facilitates transmission of laser outputs from a DFB laser array, through a coupler, to a single mode optic fiber, in a manner which does not have the alignment, coupling, processing and yield problems associated with active region and passive region integration that are present according to contemporary practice.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a PIC comprising a DFB laser array and an active waveguide coupler, which receives outputs from the DFB laser array. The active waveguide coupler facilitates high output coupling of the outputs of the DFB laser array to a single mode optical fiber, optionally with enhanced linewidth and noise characteristics. The fabrication process of the present invention decreases processing complexity and enhances yield.
These, as well as other advantages of the present invention, will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.